Battery charging systems typically utilize a step-down topology, with an input supplied by a power supply adapter, and an output coupled to a single-cell battery, or multi-cell batteries. A universal serial bus (USB) power supply system usually has a rating power supply voltage of 5V. A single-cell lithium battery can employ such step-down topology to charge the single-cell battery. Step-up topology can be used in battery management systems with series connected multi-cell lithium batteries. However, in some circumstances, such as high input current or short-circuit at the output, if relatively large current is used directly for charging, the electronic device and/or its battery can be damaged.